Crystalline Boron Carbide Encapsulated Into Carbon Nanoclusters from ARC-Discharge Soot
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2) the spatial distribution of the deposit is strongly shifted in favor of deposition of multiwalled polyhedra and tubules containing boron carbides in the soot rather than in the slag. This is in contrast to the conventional arc discharge. The apparent effect of boron on the products of the arc discharge is in agreement with the strong enhancement of graphitization by the presence of boron in pyrolytic carbon observed before [22]. Apparently, this enhancement overcomes the more gentle process environment in the soot that does not stimulate the formation of multiwalled cages and encapsulation if no boron is present. EXPERIMENTAL PROCEDURE The samples were retrieved from the soot and the cathodic deposits of an arc-discharge sustained by 43 V at 70 amps dc. under a helium atmosphere of 550 Torr. The boron composite anode, of cylindrical shape of 0.64 cm diameter and 30 cm long, consisted of a homogeneous mixture of 17 wt.% pure boron powder and graphite cement. The cathode was a cylindrical graphite rod of 0.95 cm diameter and 3 cm long. The gap between the electrodes was maintained at approximately 1 mm. At the end of the deposition, the soot was collected from walls of the reactor vessel, and the cathodic deposit was broken off from the cathode. For the transmission electron microscopy (TEM) observation, the carbonaceous deposits were ground into powder and dispersed in ethanol. After sonicating for approximately five minutes, drops of the black suspension were deposited on a holey-carbon copper grid and the solvent allowed to evaporate. The samples were then examined in a Hitachi 8100 TEM at 200 keV. The chemical composition of the encapsulants was analyzed using the parallel detection electron energy loss spectrometer (PEELS) attached to the microscope. Electron diffraction patterns obtained with the electron images were used to determine the crystalline phase of the boron compounds in the samples. RESULTS AND DISCUSSION TEM observation of the soot prepared from the boron composite anode revealed a morphology never observed before in soot samples which normally consist of crystalline C60, C70 and amorphous carbon. In particular, the characteristic graphitic fringes of 0.34 nm spacing both in forms of connected networks or in polyhedra cages are normally absent. In contrast, the soot prepared from the boron composite anode contained in the soot numerous polyhedra clusters coated with graphitic carbon as shown in a low magnification TEM image in Fig.1. The particles
Fig. 1 A low magnification TEM image of encapsulated polyhedra particles in the soot 48
have diameters ranging from 10 to 30 nm and consist of a crystalline core surrounded with four to ten layers of graphitic stacks. Arrows indicate some of the particles whose graphitic layers are not fully developed. Again in low magnification, Fig.2 below displays the morphology of the
Fig. 2 A low magnification TEM image showing the morphology of the soot majority of the soot which consists of connected graphitic "ribbon", a structure never observed before in
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